Oscillator-modulator system



March 31, 1942. H. A. WHEELER 2,278,159

OSCILLATOR-MODULATOR SYSTEM Filed July 12, 1953* INVENTOR flamZzl A Week) ATTORN YS Patented Mar. 31, 1942 OSCILLATOR-MODULATOR SYSTEM Harold A. Wheeler, Great Neck, N. Y., assignor to Haz'eltine Corporation, a corporation of Delaware 7 Application July 12, 1933, serial No. 679,997

Claims.

This invention relates to electric wave transmission and more particularly to means for controlling the transmission of signal waves in a vacuum tube.

In the use of vacuum tube transmission systems, it is common practice to impress an alternating voltage upon the input of a tube, and to apply or introduce another oscillatory voltage so that modulation occurs in the tube. A tube used in this manner is known as a modulator; and the modulation products at its output may be transmitted or radiated, as by being impressed upon a radiating antenna.

The present invention is directed to an improvement in a vacuum tube modulator system. This improvement results from forming a virtual cathode in a vacuum tube, and utilizing the virtual cathode as a source of electrons for the signal translating or modulating section of the tube. The properties of the virtual cathode, particularly its electron density, and hence its available emission, are varied periodically in order to modulate the signal. The arrangement is, therefore, designated an emission valve modulator system.

, The invention is pref rably effected by utilizing a tube having a cathode, an anode and an output control grid therebctween upon which is impressed a source of alternating voltage. Whereas the cathode actually furnishes the electron emission for the entire tube, the output control grid operates more directly on the virtual cathode, which is a cloud of electrons transferred from the cathode to a position nearer the output control grid. The virtual cathode is formed just outside of a screen located between the cathodeand the output control grid, and held at a positive voltage relative to the cathode. The output control grid is hereinafter called the outer control grid because of its location beyond the virtual cathode. 'The number of electronsreaching the virtual cathode is controlled, and hence its density is caused to fluctuate, by the valve action of an inner control grid located be tween the actual cathode and the virtual cathode.

An important embodiment of this invention lies in its application to a combined oscillator andmodulator. In this arrangement, the screen is inserted in the space path between the inner and outer control grids, the screen acting also as the anode of an oscillator. Thus, the device has, in effect, two anodes with the outer or modulator control grid situated therebetween. An appropriate oscillatory circuit is associated with the cathode, the inner control grid and the screen to produce the oscillations. The effectthus produced in the space path causes by its valve action a modulaton of the signal voltage impressed upon the outer or modulator control grid; there is thus provided an oscillator-modulator requiring the use of onlyone tube having a single cathe ode in a unitary electrode structure. An advantage of this form of oscillator-modulator lies in the fact that only a single tube is required to perform the two functions of producing oscilla tions and modulation.

An important advantage of the emission valve modulation system, according to this invention,

, resides in the fact thatit is possible to apply practically any biasing potential to the outer control grid or to any other electrode beyond the virtual cathode without materially affecting the source of oscillations. The system, therefore, readily lends itself to the control of its output by means of an adjustable biasing potential applied in this manner.

A feature'of this invention is its use in a radiofrequency transmitting system. In such a system, radio-frequency oscillations are modulated by the audio-frequency which is to be superimposed uponthe radio-frequency carrier wave. This function can be performed by impressing the audio frequency upon one of the control grids of the tube, the radio-frequency carrier voltage being impressed upon the other control grid. The modulated carrier wave at the anode circuit may then be transmitted or radiated by coupling the anode to a suitable radiating antenna.

It is advantageous that the modulator control grid be constructed to provide a; distortionless, that is, linear modulation for all percentages of modulation up tosubstantially Distortionless modulation is obtained when there is an approximately linear relationship between grid bias andmutual conductance. A bias voltage should be impressed upon the grid of the value equal'to that at the mid-point of the straight line characteristic.

It is also desirable that the control grid for the radio-frequency oscillations be constructed to provide distortionless amplification. This results when there is a linear relationship between grid voltage and anode current. On this grid,

also, there should be placed a biasing voltage of that value which is at the mid-point of the linear characteristic. quency grid may be operated with an average bias beyond the cutoff, in the case of class C amplification, with high efficiency, any harmon- Alternatively, the carrier freics generated being suppressed in the output circuit.

The various embodiments of the invention may conveniently be carried out by the use of an emission valve modulator tube having five or siX, or more, electrodes.

The output of the modulator system can be greatly increased by the use of an outer positive screen in the modulator section of the tube, between anode and outer control grid.

In accordance with the invention as embodied in a modulated radio-frequency transmitting system, an oscillator-modulator comprises a vacuum tube having an audio-frequency input circuit and. a carrier-frequency output circuit, the vacuum tube having a space current path in which are included at least two grids. The first of these grids has a source of carrier-frequency voltage connected thereto, and the second of the grids has the aforesaid input circuit connected thereto. The oscillator-modulator comprises means including the aforesaid second grid of nonuniform construction or variable mesh for imparting to the tube an approximately linear relationship between the voltage on the second grid and the mutual conductance from the first grid to the anode over a large range of voltages on the second grid, whereby distortionless modulation is effected over a large range of modulation percentages. The modulator'also includes means for coupling the output circuit to a radiating antenna.

The above and other features and advantages of the invention will become more apparent from the following detailed description when considered with the accompanying drawing of which:

Figure 1 is a schematic circuit diagram showing a generalized form of the invention.

Figure 2 illustrates a radio-transmitting system utilizing a hexode tube as an oscillatormodulator; and

'Figure 3 illustrates a somewhat similar system in which the hexode tube acts only as a modulator.

Figure 1 shows a circuit diagram of an oscillator-modulator system which may be used in a signaling system. The system comprises a hexode, or six-electrode tube I0 provided with an electron-emitting cathode I and an anode 6. In

the space path between the cathode and anode there are placed four grid-like electrodes 2, 3, 4 and 5, these four electrodes being situated in the space path at successively increasing distances from the cathode toward the anode. The schematic diagram shows the cathode and anode located at opposite sides of the tube, but this conventional form of illustration is only for the purpose of schematic representation. Any tube actually used for this purpose would be likely to have an orthodox construction, such as a centrally located cathode with helical grids surrounding the cathode, the successive grids having increasing diameters with respect to each other, and an anode in the form of a cylindrical plate surrounding the grid structures.

A source of alternating voltage, designated S1, is connected between the outer grid 4 and the cathode I, and a second source of alternating voltage S2 is connected between the inner grid 2' and the cathode. If desired, either the source S1 or the source S2, or both, may be constituted by a feedback oscillatory system associated with the tube itself. A convenient form of self-oscillatory, or feedback system, for this purpose is shown in Fig. 2 wherein an oscillatory feedback system is associated with the electrodes 2 and 3. The output circuit of the tube includes a tuned circuit comprising a coil and condenser in parallel. This tuned circuit is designated 0 and is connected between the anode and ground. Output terminals 0' and O" are connected, respectively, at opposite sides of the tuned output circuit 0. These output terminals may be connected to any succeeding apparatus to which it is desired to transmit the signals.

For the purpose of rendering the system operative, there are provided in series between the anode and ground, sources II and I2 of operating voltage making anode 6 and screens 5 and 3 positive relative to the cathode. There is also provided a voltage source l3 in series between grid 4 and ground, making grid 4 negative relative to the cathode. Grid 2 is placed at a negative potential by a battery 9.

The positive voltage on screen 5 should generally be less than that on anode 6 with respect to the cathode. This screen may be omitted, but in any embodiments of the invention its inclusion serves to improve the general operation.

In the operation of the above-described system, electrons emitted from the cathode l are attracted through the meshes of inner grid 2 to the screen 3, by virtue of the positive voltage applied to the latter. The electrons approaching screen 3 are traveling at high speed and most of them, therefore, pass on through the screen and approach the outer grid 4 which is ordinarily negative. The grid 4, therefore, serves to retard the progress of the electrons and many of them i are attracted back toward the positive screen 3.

This cloud of retarded electrons hovering between the electrodes 3 and 4 is said to constitute a virtual cathode with respect to the succeeding electrodes 4, 5 and 6 of the modulator, because of the fact that electrons can be easily drawn away from the cloud in the same manner that they were originally drawn away from the actual cathode. The virtual cathode and its approximate position is indicated by the dotdash line 1, it being understood that the line I does not represent a physical element of the tube. The positive potentials of the anode B and screen 5 serve to attract electrons from the virtual cathode to the anode, through the outer grid 4 and the screen 5, in the usual manner. The tube, therefore, has in effect two anodes, namely, elements 3 and 6, with the inner grid controlling the entire emission from cathode l and the space current to both anodes. It will, therefore, be observed that the electrodes 4, 5 and 6 together with the'virtual cathode 1, function as an ordinary signal translating vacuum tube of which the output circuit 0 is connected between anode 6 and the virtual cathode (in so far as alternating currents are concerned). From the above-described operation, it is observed that the output controlling electrodes are grids 2 and 4.

Modulation results in the system in the following manner. When the grid 2 is only slightly negative, or is somewhat positive, there is an abundant supply of electrons at the virtual cathode 1 available to supply an electron stream in the modulator section of the tube. When the grid 2 swings considerably negative, the virtual cathode, and therefore the anode 6, are momentarily deprived of their electron supply, thereby interrupting the output current. It will be observed, then, that the output current of the mod ulator varies in accordance with the oscillations of source S2.

This is the valve action by which the .alternating voltage .of source S2 modulates thealternating voltage 'ofsource .S1 in the tube, producingzthe-welhknown beat note in the output circuit, having a frequency equal to the sum or difference'of :the source frequencies.

The elementsof the tube, other than the actual cathode, maybe consideredtobe-situated in two groups: (12) elements 14,5 and 6 associated with source S1; and (2) elements 2 and3 associated with source ,Sz. Theelements within each group are mutually adjacent, but all the elements of group .1 lie beyond =thoseof group 2.

'It is possiblerto controlthe output or conver-- sion gain of .the modulatorover a wide range of intensities "by selecting the negative bias applied to the-outer control .grid 4. It will be observed that this .type of-control does not materially affectzthesbehavi'orof theoscillatory portion of the system becausethe .grid '4, regardless of its potential, is incapable of cutting off the electron current to the oscillator screen 3.

Figure 2 shows an oscillator-modulator system,.somewhat similar to that of Figure 1, used in a modulated radio-frequency transmitting system. The tube .ill shown in Figure 2 is similar to the tube glll of Figure 1, and includes a space current path betweenthe cathode l and anode 6 in which are included three grids 2, 3 and 4. Similar numerals are used to designate elements which :are. similar in the two figures. In place of the source of alternating voltage S2 of Figure 1, the tube of Figure 2 is arranged to oscillate by means of anoscillatory system arranged between screent and cathode I. The oscillatory system comprises .a coil I5, in parallel with condenser I6, connected at one end to ground (and to the oathode) and .coupled at .the other end through a blocking condenser .llto the inner grid 2. For the purpose :of producing the oscillations, there is connected in series between the screen 3 and battery 12 a coil l8coupled to the coil 15 of the oscillatorycircuit. The coil 18 feeds back sufficient energy to the grid 2, through the tuned circuit I5, IE, to sustain the oscillations at a sufiiciently high amplitude. Thus, the electrode 3 and the circuit elements l4 to IE, inclusive, comprise a source of carrier-frequency voltage connected to the first grid '2'. As in the arrangement of Figpl, .the'tuned circuit comprises a carrier-frequency output circuit for the vacuum tube It). I

The instant arrangement also includes an audio-frequency input circuit comprisinga microphone 49 which is coupled to and applies an audio frequency voltage to the outer or third grid 4 through an input transformer 20. This audio frequency on the outer grid serves to modulate the oscillations within the tube.

Means is included for coupling the output circuit O to a radiating antenna 2|, this means comprising an output transformer 22.

The elements of the tube, other than the actual cathode, may be considered to be situated in two groups:

(1) Elements 4, and 6 associated with the source of modulation or audio frequencies; and

(2) Elements 2 and 3 associated with the oscillatory carrier-frequency system.

The eiements within each group are mutually adjacent but all the elements of group 1 lie beyond those of group 2. v

The operation of the system can often be improved by constructing the modulator, or audio frequency, control grid to provide a distortionless, that is, linear modulation for all percentages of modulation up to substantially Such .distortionless modulation :is obtained over any range of zmodulationpercentages :for which there is anapproximatelylinearrelationship between .grid-bias and mutual conductance. The biasing voltage applied to this grid should be that valuewhich lies approximately at .themidpointof the linear curve. This desirable type of grid characteristic requires a grid'jhavinga nonuniform grid mesh throughout its extent. The exact variation'of grid spacings can best be determined by trial for any particular design of tube.

Itis often desirable that the'inner control grid 2, which controls the carrier-frequency oscillations, be constructed to provide distortionless amplification. As is well known, distortionless amplification can be arrived at by'the selection of the grid spacing; and the proper spacing can readily be arrived at'by trial and error.

lncase the system is-operated as a' class C amplifier, the carrier-frequency grid will be operated with-a considerably higher negative bias, beyond the-cutoff, the plate circuit efficiency 'beingquite high in this case;

From the above description of the Fig. 2 arrangement, it will be seen that this modification of the invention includes means including a nonuniform construction or variable mesh of the grid 4 for imparting to the tube l0 an approximately linear relationship between the voltage on the grid 4 and the mutual conductance from the first grid 2 to the anode 6 over a large-range of voltages onthe second grid, whereby distortionless modulation is effected over alarge range of modulation percentages. 'There is also included means including the first grid 2 for imparting to the tube It) a linear relationship between the first grid voltage and the space current of the tube, whereby distortionless amplification is effected. v i

Figure 3 "shows a variation of the system in Figure 2, the tube It in Figure Bacting only as a modulator. In thisarrangement, the tube does not serve to produce oscillations, but these are applied from an externalsource -23 andapplied to the outer control-grid 4 which, in this case, becomes the carrier or radio-frequency control grid. The audio frequency from the'microphone i9 is impressed upon the inner grid 2.

Aside from the interchanging of theradio-frequency and audio frequency on control grids 2 and 4, the action of the tube is otherwise quite similar to that in the system of Figure 2, the modulator tube being coupled to the antenna 2| in the same way. The functions of the control grids are interchanged, and also the design criteria discussed for Fig. 2, which determine the non-uniform grid spacing.

Although pentode tubes are operative (the screen 5 being omitted), best results are obtainable with special six-electrode tubes or hexodes. Referring to Figs. 2 and 3, grid 2 may be of uniform or varying mesh. Uniform mesh is preferred for class C operation. Screens 3 and 5 are preferably of fine mesh.

The voltages on grid 4, screen 5 and anode B are preferably chosen to hold the average anode current less than one-half the average screen current, or less than one-third the average cathode current, the cathode current being substantially the sum of the screen and anode currents. This relation provides that changes in anode current have a negligible effect on the performance of the oscillator circuit.

'I claim: l 1 I l a 1.'A m'odulated radio-frequency transmitting system comprising a vacuum tube having an audio-frequency' input circuit and a carrier-frequency output circuit, said vacuum tube having a space-current path in which are included at leastv two grids, a first of said grids having a source of carrier-frequency voltage connected thereto, the second of said grids having said input circuit connected thereto, means including a non-uniform construction of said second grid for imparting to said-tube an approximately linear relationship between the voltage on said second grid and the mutual conductance from said first grid to said anode over a large range of voltages on said second grid, whereby distortionless modulation is efiected over a large range of modulation percentages, and means for coupling said output circuit to a radiating antenna,

2. A modulated radio-frequency transmitting system comprising a vacuum tube having an audio-frequency input circuit and a carrier-frequency output circuit, said vacuum tube having a space current path in which are included at least two grids, a first of said grids having a source of carrier-frequency voltage connected thereto, the second of said grids having said input circuit connected thereto, means including a non-uniform construction of said second grid for imparting to said tube an approximately linear relationship between the voltage on said second grid and the mutual conductance from said first grid to said anode over a large range of voltages on said second grid, whereby distortionless modulation is effected over a large range of modulation percentages, means including said first grid for imparting to said tube a linear relationship between said first grid voltage and space current, whereby distortionless amplification is effected, and means for coupling said output circuit to a radiating antenna.

3. In a modulated radio-frequency transmitting system, an oscillator-modulator comprising a vacuum tube having a cathode, an anode and a first, second and third grid positioned in the space path successively from said cathode to said anode, an audio-frequency input circuit connected between said third grid and said cathode, an output circuit connected between said cathode and said anode, a tuned circuit connected to said cathode and coupled tosaid first and second grids for producing sustained radio-frequency oscillations, said third grid having a non-uniform mesh to provide an approximately linear relationship between'the voltage on said third grid and the mutual conductance from said first grid to said anode over a wide range of voltages on said third grid, whereby distortionless modulation is effected over a large range of modulation percentages, and means for coupling said output circuit to a radiating antenna.

4. In a modulated radio-frequency transmitting system, an oscillator-modulator comprising a vacuum tube having a cathode, an anode and a first, second and third grid positioned in the space path successively from said cathode to said anode, an audio-frequency input circuit connected between said third grid and said cathode, an output circuit connected between said cathode and said anode, a. tuned circuit connected to said cathode and coupled to said first and second grids for producing sustained radiofrequency oscillations, means including said'first grid for imparting to said tube :a linear relationship between grid bias and space current, whereby distortionless amplification is effected, means including a non-uniform construction of said third grid for imparting to said tube an approximately linear relationship between the voltage on said third grid and the mutual conductance from'said first grid to said anode overa large range of voltages on said third grid, whereby distortionless modulation is effected overa large range of modulation percentages, and means for coupling said output circuit to a radiating antenna.

5. A modulated radio-frequency transmitting system comprising a vacuum tube having an audio-frequency input circuit and a carrier-frequency output circuit, said vacuum tube having a space current path'in which are included at least two grids, a first of said grids having a source of carrier-frequency voltage connected thereto, the second of said'grids having said input circuit connected thereto and having a variable mesh for imparting to said tube an approximately linear relationship between the voltage on said second grid and the mutual conductance from said first grid to said anode over a large range of voltages on'said second grid, whereby distortionless modulation is effected over a large range of modulation percentages, and means for coupling said output circuit to a radiating antenna.

HAROLD A. WHEELER. 

